FIELD OF THE INVENTION
[0001] In recent years, a technology of casting a thin cast strip having a strip thickness
of 10 mm or less directly from a steel melt was developed and tested on an industrial
scale. This new technology provides a process of producing a cold-rolled thin sheet
product, in which a hot rolling step is either simplified or omitted, and thereby
attracts considerable attention and is expected from the view point of saving energy
and cost.
[0002] Processes of producing a thin sheet, including the above process, will hereinafter
be referred to as "STC processes" (Strip Casting Process). In contrast, processes
of producing a cold-rolled thin sheet product which includes the steps of continuous-casting
a 100 mm or more thick steel slab, hot-rolling the slab to a several millimeters thick
hot-rolled strip, and cold-rolling the hot-rolled strip, will be referred to as a
conventional process.
[0003] The present invention relates to a process of producing a thin cast strip having
a high toughness, particularly a thin cast strip of a Cr-stainless steel containing
Nb, Ti, Al, etc., by an STC process.
BACKGROUND ART
[0004] Conventionally, Cr-stainless steels were produced by a conventional hot rolling-process
which included casting a slab and hot-rolling the slab. This process had a problem
that ridging (or roping) occurs in the cold-rolled thin sheet products due to a texture
established during hot rolling. Then, trials were conducted in which the STC process
was used to cast a thin cast strip for producing a thin sheet product in which no
ridging occurs. For example, Japanese Unexamined Patent Publication (Kokai) No. 62-176649
disclosed "Process of Producing Ferritic Stainless Steel Thin Strip Having No Roping".
This publication, however, did not describe the reduction in toughness which occurs
in Cr-stainless steels having a single phase structure and containing Nb, Ti, Al,
and V in an amount of from 0.05 to 1.0 wt% in total. Therefore, there remains a problem
that a cast strip of a Cr-stainless steel containing Nb, Ti, Al and V in the above-recited
total amount has too low a toughness to be cold-rolled in a subsequent step.
[0005] Japanese Unexamined Patent Publication (Kokai) No. 64-4458 entitled "Rapid-Cooled
Strip of Ferritic Stainless Steel Having High Toughness" disclosed that a cast strip
having a high toughness can be produced by controlling its columnar crystal content
to 70% or more, but did not consider the technological significance of the relationship
between the toughness of and the precipitates in the cast strips of Cr-stainless steels
containing NB, Ti, Al, and V.
[0006] The present inventors have been developing a technology of producing Cr-stainless
steel thin sheet by using an STC process. As a result, it became apparent that cast
strips have a poor toughness which causes cracking to occur during cold rolling of
SUS 430 or other steel systems in which a γ-phase is precipitated during cooling after
solidification to room temperature and a martensite phase transformed from the γ-phase
remains at room temperature.
[0007] To prevent the precipitation of γ-phase during cooling after solidification to room
temperature, the present inventors produced a thin cast strip of a Cr-stainless steel
with a controlled chemical composition having a γp value of 0% or less. The term γp
is a parameter predicting the precipitate amount of γ-phase based on the chemical
composition. However, even when a Cr-stainless steel has a γp of 0% or less, there
remains a problem that a cast strip has a poor toughness and is broken during cold
rolling when it contains one or more of Nb, Ti, Al and V in an amount of 0.05 wt%
or more in total.
[0008] The present inventors made a study and found that thin cast strips of Cr-stainless
steels containing such elements and exhibiting a poor toughness contain fine precipitates
with a size of 0.1 µm or less. It is known that such fine precipitates harden the
steel matrix and thus deteriorates the toughness.
[0009] A thin cast strip cast by an STC process contains fine precipitates of 0.1 µm or
less, probably because its speed of cooling after solidification to room temperature
is much higher than that of a slab cast by the conventional process, so that those
precipitates, which precipitate and can grow to several µm during cooling of a slab
by the conventional process, do not actually have sufficient time to precipitate and
grow but precipitate in a fine form instead in a thin cast strip cast by an STC process.
[0010] Thus, to improve the toughness of a thin cast strip of a Cr-stainless steel containing
one or more of Nb, Ti, Al and V in an amount of 0.05 wt% or more in total, it is required
that precipitates be grown to 0.1 µm or greater.
[0011] This problem occurrs in Cr-stainless steels containing Nb, Ti, Al and/or V in an
amount of 0.05 wt% or more, irrespective of the structure of a cast strip such as
the content of columnar crystals.
[0012] It also became apparent that the conventional hot-rolling process has no problem
concerning the toughness of hot-rolled and annealed sheets of the subject steels of
the present invention and that the problem is specific to STC processes.
DISCLOSURE OF THE INVENTION
[0013] The object of the present invention is to solve the above-discussed problem in STC
processes.
[0014] To achieve the object according to the present invention, there is provided a process
of producing a thin strip of a Cr-stainless steel having a high toughness, characterized
by the steps of: casting a thin cast strip of a Cr-stainless steel having a thickness
of 10 mm or less, the steel containing 13-25 wt% of Cr, 0.05-1 wt% of one or more
of Nb, Ti, Al and V in terms of a total amount, 0.03 wt% or less of C, 0.03 wt% or
less of N, and 0.3-3.0 wt% of Mo in accordance with need, and having a γp value of
0 % or less, γp being defined as γp(%) = 420C + 470N + 23Ni + 9Cu + 7Mn - 11.5Cr -
11.5Si - 12 Mo - 23V - 47Nb - 49 Ti - 52Al + 189 (respective elements in wt%); hot-rolling
the thin cast strip in a temperature range of from 1150 to 950°C at a reduction in
thickness of 5 to 50 % to form a thin strip; either slowly cooling the thin strip
at a speed of 20 °C/sec or less or holding the thin strip for 5 sec or more, in a
temperature range of from 1150 to 950°C, or passing the thin strip through a heat
treatment furnace held at a temperature of from 1150 to 950°C for 5 sec or more; and
then coiling the thin strip at a temperature lower than 700 °C.
[0015] According to the present invention, the chemical composition of steel is numerically
limited as mentioned above for the following reasons.
Cr: 13-25 wt%
[0016] Cr effectively improves the corrosion resistance, the oxidation resistance at high
temperatures, and other properties of a steel. To ensure these properties at least
to an extent necessary in the Cr-stainless steels for usual applications, the Cr content
must be 13 wt% or more. This content is also a minimum amount necessary to control
the γp value to be 0% or less by adjusting the contents of other components. On the
other hand, the Cr content must be 25 wt% or less because the toughness is significantly
reduced when the Cr content is more than 25 wt%.
γp: 0% or less
[0017] γp is a parameter for calculating the amount of precipitated γ-phase based on the
chemical composition. Any precipitated γ-phase is transformed to martensite phase
during cooling to room temperature and the hard martensite phase significantly deteriorates
the toughness. Therefore, to prevent γ-phase from being precipitated, γp is limited
to 0% or less.
[0018] γp is defined by the formula: γp(%) = 420C + 470N + 23Ni + 9Cu + 7Mn - 11.5Cr - 11.5Si
- 12 Mo - 23V - 47Nb - 49 Ti - 52Al + 189 (respective elements in wt%).
One or more of Ti, Al, Nb, V: 0.05 to 1.0 wt% in total
[0019] Generally, Ti, Al, Nb, and V are occasionally added to a ferritic stainless steel
in order to improve the corrosion resistance and the formability. These elements,
however, are precipitated in the form of fine particles in thin cast strips solidified
by rapid cooling and deteriorate the toughness of the cast strip. When contained in
an amount of less than 0.05 wt%, these elements are harmless to the toughness, but
when present in an amount of 0.05 wt% or more, fine particles of about 0.1 µm are
precipitated and deteriorate the toughness. Thus, the present invention is directed
to an improvement of the toughness of Cr-stainless steels containing one or more of
Ti, Al, Nb, and V in an amount of 0.05 wt% in total as specified as a lower limit
in the claims. The upper limit is specified as 1.0 wt%, because an amount greater
than 1.0 wt% does not further improve the corrosion resistance and the formability
under usual environmental conditions.
C, N: 0.030 wt% or less
[0020] Generally, C and N cause Cr to precipitate as a carbonitride on grain boundaries,
and thereby, deteriorate the grain boundary corrosion resistance and the toughness.
Therefore, the contents of these elements must be as small as possible and limited
to 0.030 wt% or less.
Mo: 0.3 to 3.0 wt%
[0021] Similar to Cr, Mo effectively improves corrosion resistance. Thus, Mo is present
together with Cr to improve the corrosion resistance in an amount of 0.3 wt% or more
to ensure this effect but must not be more than 3 % because greater amounts would
induce the embrittlement due to the precipitation of sigma and chi phases.
[0022] The cast strips are hot-rolled and cooled under the conditions specified for the
following reasons.
[0023] STC process uses rapid cooling of cast strips after casting and therefore there is
only a little time for the precipitation and growth of the precipitated particles.
Therefore, a heat treatment for the precipitation and growth is necessary. Because
a thin cast strip has only a few precipitation sites, the heat treatment must be carried
out at a high temperature for a long time to induce the precipitation and growth.
To perform such a heat treatment on the cast strip immediately after casting, there
is a problem that a long and large heat treatment line is necessary.
[0024] Thus, it is desired to provide a technology which enables the precipitation and growth
to occur in a short time. Introduction of dislocations providing nuclei for precipitation
effectively facilitates the precipitation. Namely, hot rolling in a precipitation
temperature region effectively promotes precipitation. After the hot rolling to promote
the precipitation, a slow cooling or an isothermal holding is performed to cause the
precipitates to grow. These treatments ensure the precipitation and growth of the
precipitates in a short time and render any precipitates in a cast strip harmless.
[0025] Cast strips are hot-rolled at a temperature of from 1150 to 950 °C and at a reduction
in thickness of 5 % or more, based on the following experimental results.
[0026] The present inventors carried out a laboratory experiment, in which a Fe-19wt%Cr-0.60wt%Nb-0.015wt%C-0.015wt%N
steel was cast into 3 mm thick cast strips, which were then hot-rolled in a temperature
range of from 1200 to 800 °C with a reduction in thickness of 3 to 50 % to form thin
strips. The hot-rolled thin strips were then passed through a heat treatment furnace
held at 1100°C for 10 sec, were secondarily cooled at 100 °C/sec to 500 °C, and were
then coiled. The thin strips were subjected to a Charpy impact test at room temperature
to estimate the toughness. The Charpy impact test was performed by using a specimen
with the thickness of the thin strip.
[0027] The results are summarized in Fig. 1. The cast strips had a high toughness when hot-rolled
at a reduction in thickness of 5% or more and at a temperature of from 950 to 1150°C.
It is believed that carbonitrides were not rendered harmless because carbonitrides
are not precipitated at temperatures above 1150°C and because carbonitrides, even
if precipitated, do not grow fast at temperatures below 950°C.
[0028] The hot rolling must be performed at a reduction in thickness of 50 % or less, because
higher reductions cause spill-like defects to occur.
[0029] The hot-rolled strip is either held for 5 sec or longer or slowly cooled at 20 °C/sec
or less, in a temperature range of from 1150 to 950°C. These conditions are determined
by the following experiment.
[0030] The present inventors carried out an experiment, in which a Fe-19wt%Cr-0.60wt%Nb-0.015wt%C-0.015wt%N
steel was cast into 3 mm thick cast strips, which were then hot-rolled at 1000°C at
a reduction in thickness of 10%. The hot-rolled strips were heat-treated under different
conditions, secondarily cooled at 100°C/sec to 500 °C, and coiled. The hot-rolled
strips were subjected to a Charpy impact test at room temperature to estimate the
toughness. The Charpy impact test was performed by using a specimen with the thickness
of the thin strip.
[0031] The results are summarized in Figs. 2 to 4. The hot-rolled strips had a high toughness
when held for 5 sec or more or slowly cooled at 20°C/sec or less in a temperature
region of from 1150 to 950°C. Poor toughness was obtained under other conditions,
probably because carbonitrides did not grow sufficiently.
[0032] It is advantageous for process control that the heat treatment after hot rolling
is effected by passing the hot-rolled strip through a heat treating furnace held at
a temperature of from 1150 to 950 °C. In this case, the hot-rolled strips also had
a high toughness after being passed through the furnace for 5 sec or more in a temperature
region of from 1150 to 950°C.
[0033] Stainless steels containing elements such as Ti and Nb, when held at 700 to 900°C
for a long period of time, have a poor toughness due to precipitation of very brittle
intermetallic compounds (Laves phase). Thus, the strip must be coiled at a temperature
of lower than 700 °C.
[0034] This control of precipitates by hot rolling and heat treatment under the above-stated
conditions was proved to be effective not only with Nb-containing steels but also
with Ti- or Al-containing steels.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] Figure 1 is a graph showing the relationship between the hot rolling conditions of
the cast strip and the cast strip toughness.
[0036] Figure 2 is a graph showing the relationship between the heat treatment conditions
after hot rolling and the cast strip toughness.
[0037] Figure 3 is a graph showing the relationship between the heat treatment conditions
after hot rolling and the cast strip toughness.
[0038] Figure 4 is a graph showing the relationship between the heat treatment conditions
after hot rolling and the cast strip toughness.
BEST MODE FOR CARRYING OUT THE INVENTION
Example
[0039] Various Cr-stainless steels having the chemical compositions shown in Table 1 within
the claimed range of the present invention were melted to provide 10-ton melts, which
were then cast to thin cast strips having a thickness of 3 mm in a water-cooled twin-drum
caster. The cast strips were hot-rolled in the temperature range of from 1150 to 950°C
at different reductions in thickness of from 5 to 50 %, were held, or slowly cooled,
for 5 sec in the temperature range of from 1150 to 950°C, and were coiled in the form
of a thin strip.
[0040] For comparison, Cr-stainless steels having the chemical compositions shown in Table
1 as comparative examples were also cast in a similar manner. The cast strips were
hot-rolled, heat-treated after the hot rolling, and coiled under the respective conditions,
at least one of which was outside the claimed range, to produce thin strips.
[0041] As can be seen from Table 2, the thin strips produced by the present inventive process
had a high toughness of 2 kgf-m/cm² or greater at 0 °C whereas the thin strips produced
by the comparative process had too low a toughness of less than 2 kgf-m/cm² to carry
out the subsequent step of cold rolling.

INDUSTRIAL APPLICABILITY
[0042] As hereinbefore-described, the present invention provides a process of producing,
by an STC process, a thin cast strip of a Cr-stainless steel having a high toughness,
thereby providing an extremely great technological and economical advantage.
1. A process of producing a thin strip of a Cr-stainless steel having a high toughness,
characterized by the steps of: casting a thin cast strip of a Cr-stainless steel having
a thickness of 10 mm or less, said steel containing 13-25 wt% of Cr, 0.05-1 wt% of
one or more of Nb, Ti, Al and V in terms of a total amount, 0.03 wt% or less of C,
0.03 wt% or less of N, and 0.3-3.0 wt% of Mo in accordance with need, and having a
γp value of 0 % or less, said γp being defined as γp(%) = 420C + 470N + 23Ni + 9Cu
+ 7Mn - 11.5Cr - 11.5Si - 12 Mo - 23V - 47Nb - 49 Ti - 52Al + 189 (respective elements
in wt%); hot-rolling said thin cast strip in a temperature range of from 1150 to 950
°C at a reduction in thickness of 5 to 50 % to form a thin strip; either slowly cooling
said thin strip at a rate of 20 °C/sec or less or holding said thin strip for 5 sec
or more, in a temperature range of from 1150 to 950 °C ; and then coiling said thin
strip at a temperature lower than 700°C.
2. A process of producing a thin strip of a Cr-stainless steel having a high toughness,
characterized by the steps of: casting a thin cast strip of a Cr-stainless steel having
a thickness of 10 mm or less, said steel containing 13-25 wt% of Cr, 0.05-1 wt% of
one or more of Nb, Ti, Al and V in terms of a total amount, 0.03 wt% or less of C,
0.03 wt% or less of N, and 0.3-3.0 wt% of Mo in accordance with need, and having a
γp value of 0 % or less, said γp being defined as γp(%) = 420C + 470N + 23Ni + 9Cu
+ 7Mn - 11.5Cr - 11.5Si - 12 Mo - 23V - 47Nb - 49 Ti - 52Al + 189 (respective elements
in wt%); hot-rolling said thin cast strip in a temperature range of from 1150 to 950
°C at a reduction in thickness of 5 to 50 % to form a thin strip; passing said thin
strip through a heat treatment furnace held at a temperature of from 1150 to 950°C
for 5 sec or more; and then coiling said thin strip at a temperature lower than 700
°C.